1. Field of the Invention
In general, the present invention relates to a compound ring network typically comprising two networks such as a UPSR (Uni-directional Protection Switch Ring) and a BLSR (Bi-directional Line Switch Ring) connected to each other. In particular, the present invention relates to a path switching method and a path switching apparatus having a function for switching from a working path to a protection path starting from two adjacent input nodes (and ending at a common path terminating node) as a normal line (an active path) by using a path switch in such a compound ring network.
2. Description of the Related Art
First of all, as conventional technologies, the BLSR and the UPSR are explained. As described in Bellcore's GENERIC REQUIREMENTS GR-1230-CORE, Issue 1, the BLSR is a ring network for connecting a plurality of nodes to each other by using transmission lines to form a ring-like shape wherein each two nodes thereof are connected by one path and, in the event of a failure occurring in a transmission line accommodating the path, a path route is changed to heal the path.
FIGS. 1 and 2 are diagrams showing configurations of path routes in a normal operation and in the event of a failure respectively in an example of a two-fiber BLSR. In particular, FIG. 1 is a diagram showing five nodes, Node-A to Node-E denoted by reference numerals 801 to 805 respectively, connected by transmission lines 806 and 807 wherein a path 808 is established between the node 801 and the node 803. In this state, assume that a failure 809 occurs on the transmission line 806 between the nodes 802 and 803 as shown in FIG. 2, causing the following switching to be carried out to heal the failing path. To be more specific, the failure 809 occurs on the transmission line 806 between the nodes 802 and 803 as shown in FIG. 2. In the event of such a failure, all paths are returned at the node 802, a node located immediately before the failure 809 occurring on the transmission line 806, establishing new path routes till the path terminating node 803 (Node-C) of the returned path through the transmission line 807. To be more specific, the path 808 shown in FIG. 2 is returned at the node 802, being continued by a newly established path 810 that ends at the node 803. In this way, the path is switched at the node 802 from the transmission line 806 to the transmission line 807, healing the original failing path 808.
As described in Chapter 3 of Bellcore's GENERIC REQUIREMENTS GR-1400-CORE, Issue 1, on the other hand, the UPSR is a network for connecting a plurality of nodes by using transmission lines to form a ring wherein each two nodes thereof are connected by two paths: a working path and a protection path.
FIG. 3 is a diagram showing the configuration of an example of the UPSR. In this example, the counterclockwise and the clockwise paths are working and protection paths respectively. To begin with, signal transmission from a node 701 to a node 703 is explained. Normally, both a working path 709 and a protection path 710 are established. At the node 703 at the end of the established paths, path-alarm detect units 711 and 712 are provided for the working and protection paths 709 and 710 respectively. In addition, the node 703 is also provided with a path select unit 713. At the present time, the path select unit 713 selects the working path 709.
Assume that a failure occurs on the working path 709 between the nodes 701 and 702 shown in FIG. 3. In this case, the failure is detected by the path-alarm detect unit 711 at the node 703. Detecting the failure, the path-alarm detect unit 711 controls the path select unit 713 to select the protection path 710 in order to recover the path from the failure.
A concrete path switching method for inter-ring connection configurations is disclosed in U.S. Pat. No. 5,390,164 with a title "Ring Interworking between Bi-directional Line-Switching Ring Transmission Systems." This document describes a switching method for maintaining communication connectivity of transmission systems in the event of a failure in a configuration comprising ring networks connected to each other by two ring nodes wherein two paths, working and protection paths, are established. Particularly, in a SONET (Synchronous Optical Network) signal, low-speed signals each known as a VT signal are multiplexed and, in addition, a multiplex frame structure for forming a high-order STS-1 signal is adopted. Thus, even if failures occur at the low-order VT-signal level, in the multiplexing process to form an STS-1 signal, pieces of information on the failures appear to be normal in controlling a switching operation. The switching method disclosed in the above reference is a switching method for reducing the probability of selecting such an abnormal low-level signal, being aimed at a case in which failures occur on the working and protection paths independently to the bitter end. Thus, the scope of the method does not include a case in which one failure affects both the working and protection lines at the same time in the inter-ring connection configuration.
A network system in which the UPSR and the BLSR described above are connected to each other is taken into consideration. According to Bellcore's GENERIC REQUIREMENTS GR-1230-CORE, in the event of path switching taking place in the BLSR due to a transmission-line failure, a time of 50 ms between the occurrence of the failure and the recovery of the line achieved through the path switching is allowed.
As shown in FIG. 3, transmission lines of a UPSR form a ring-like shape. To put it in detail, two paths in a UPSR, the working and protection paths, are set to form a circle starting from two adjacent interconnection nodes connected to a BLSR to a path terminating node as shown in FIG. 4. A difference in transmission route between the working and protection paths makes the number of nodes included in the working path different from the number of nodes included in the protection path. Thus, there is a difference in arrival time of alarm information at the path terminating node between a failure occurring on the working path and a failure occurring on the protection path. This difference in arrival time of alarm information at the path terminating node is attributed to the difference in path length caused by the difference in code count between the working and protection paths. As a result, even if a path failure with a maximum recovery time limit of 50 ms described above passes through the interconnection node at the same time, it may arrive at the path terminating node at different times.
Path switching is carried out to always select either the working and protection paths as a normal (active) path. Thus, in a case with a small difference in arrival time of failure alarm information described above, there are observed erroneous switching operations in which a selector once selects a path with a longer propagation time which appears to be the normal path before switching back later on to the path with a shorter propagation time. Particularly, in the case of a SONET signal, since low-speed paths each known as a VT signal are multiplexed and a multiplex frame structure for forming a high-order STS-1 signal is further adopted, it is quite within the bounds of possibility that erroneous switching operations described above are inadvertently carried out simultaneously on a plurality of low-speed paths.
Normally, in a transmission apparatus for handling a SONET signal, software is used for monitoring the switching state of a path. When a plurality of path erroneous switching operations described above occur at the same time, two switching operations, `switch-over` and then `switch-back`, are carried out on each path. In this case, the monitoring software must perform very complex software processing in order to monitor the switching state of each path. Assume that erroneous switching operations occur in all VT-1.5 signals accommodated in an STS-3 signal. In this case, 168 (=84.times.2) wasteful switching-state monitoring operations must be carried out in a short period of time.